The ability to degrade proteins is an essential function of all eukaryotic cells. The ubiquitin-proteasome system has evolved to play an active role in cellular quality control by selective degradation of normal or damaged proteins. The ubiquitin-proteasome system is fundamental to cell cycle control, transcriptional regulation, stress response, immune and inflammatory responses and other vital processes (Hershko and Ciechanover, 1998, Annu. Rev. Biochem, 67:425-479; Varshavsky, 1997, Trends Biochem. Sci., 22: 383-387; Hochstrasser, 1996, Annu. Rev. Genet, 30: 405-439).
Ubiquitin (Ub) is a highly conserved 76-amino acid protein. Protein degradation via the ubiquitin-proteasome pathway generally involves covalent attachment of multiple molecules of ubiquitin to the protein substrate. The protein substrate is subsequently degraded by the 26S proteasome complex, and the free ubiquitin is recycled. There are also examples of proteins whose functions appear to be regulated by ubiquitylation, although ubiquitylation does not appear to target them for degradation (Hwang et al., 2003, Mol. Cell, 11: 261-266).
The attachment of ubiquitin to many known substrate proteins is believed to occur in a series of enzymatic reactions carried out sequentially by three classes of proteins: (1) an ubiquitin-activating enzyme (E1) activates ubiquitin in an ATP-dependent manner to form a thioester bond between the carboxy-terminal Gly of Ub and a Cys residue of E1; (2) activated Ub is then transferred to an ubiquitin-conjugating enzyme (E2 or UBC) to form another thioester bond; (3) a ubiquitin ligase (E3) catalyzes or promotes, in a substrate specific manner, the transfer of Ub from the E2 to a Lys residue of the substrate protein to form an isopeptide bond. An internal Lys residue of Ub can also form an isopeptide bond with the C-terminus of another Ub to create a multi-Ub chain that serves as a targeting signal for proteasome.
In addition to ubiquitylation, proteins can be modified by attachment of ubiquitin-like proteins (such as Sentrin/SUMO or NEDD8) through distinct pathways that may have physiological roles distinct from the ubiquitylation pathway (Yeh et al., 2000, Gene, 248: 1-14). While there are at least 25 mammalian E2 family members, some poorly characterized, the number of different E3 enzymes is predicted to gross over a hundred (Weissman, 2001, Nature Reviews, 2: 169-178). E3 ubiquitin ligases come in a variety of different structural classes (such as HECT and RING finger) and act via a number of distinct pathways. So far, most E3 proteins that have been shown to interact with E2s and to mediate ubiquitylation in in vitro systems lack defined substrates other than themselves. The currently available information on E3 identification and specificity is insufficient to develop clear understanding of the role of many E3s in biological processes and disease.
The defective regulation of the ubiquitin-proteasome system manifests in diseases that range from developmental abnormalities and autoimmunity to neurodegenerative diseases and cancer (Weissman, 2001, Nature Reviews, 2: 169-178). The discovery of HECT E3s was a direct consequence of the finding that oncogenic strains of human papillomavirus (HPV) encode isoforms of a protein called E6, which specifically inactivates the tumor suppressor protein p53. E6 serves as an adaptor between p53 and an E6-associated E3 that catalyzes the ubiquitylation of p53 (Scheffner et al., 1993, Cell, 75:495-505). Mutations in the same HECT E3 enzyme are shown to give rise to Angelman syndrome, a severe neurologic disorder (Kishino et al., 1997, Nature Genet., 15: 70-73). Prominent among RING E3s is a product of breast and ovarian cancer susceptibility gene (BRCA1). Mutations in this protein are found in familial forms of breast and ovarian cancer (Brzovic et al., 1998, J. Biol. Chem., 273: 7795-7799). Among well-studied RING E3s are the oncoprotein MDM2, an E3 ligase that ubiquitylates p53 and upon overexpression may lead to cancer; the proto-oncoprotein c-Cbl which ubiquitylates growth factor receptors (Waterman et al., 1999, J. Biol. Chem., 274: 22151-22154; Joazeiro et al., 1999, Science, 286: 309-312; Yokouchi et al., 1999, J. Biol. Chem., 274: 31707-31712) and the inhibitors of apoptosis (IAP) proteins (Yang et al., 2000, Science, 288: 874-877; Hwang et al., 2000, J. Biol. Chem., 275: 26661-26664). Mutations in Parkin, another RING finger E3, are associated with juvenile Parkinson's disease (Shimura et al., 2000, Nature Genet., 25: 302-305; Chung et al., 2001, Nature Med., 7(10): 1144-1150).
One specific example of an important ubiquitylation pathway is N-End rule ubiquitylation, and especially N-End rule ubiquitylation where ubiquitylation is preceded by N-terminal segment cleavage, where the N-terminal segment comprises one or more amino acid residues. The proteolysis exposes an N-degron which comprises a destabilizing N-terminal residue plus an internal Lys residue where a multi-Ub chain is later attached. The N-terminal segment is cleaved to form an activated substrate of the Ub-dependent N-end rule pathway (activated fragment) which is recognized through exposed destabilizing N-terminal residue.
The N-end rule pathway has been the subject of several review articles; see, e.g., Varshavsky, 1996, Proc. Natl. Acad. Sci. U.S.A., 93: 12142. The ubiquitin ligase UBR1, an E3 ligase, has been shown to ubiquitylate N-end rule substrates and has two binding sites for primary destabilizing N-terminal residues. The type I site is specific for basic N-terminal residues Arg, Lys, His. The type II site is specific for bulky hydrophobic residues Phe, Leu, Trp, Tyr, and Ile. Dipeptides carrying type I or type II N-terminal residues can serve as inhibitors of ubiquitylation of the corresponding type I or type II N-end rule substrates (Gonda et al., 1989, J. Biol. Chem., 264: 16700-16712). UBR1 from yeast contains yet another substrate-binding site, which recognizes proteins for ubiquitylation through an internal recognition site on substrates; this process can be enhanced by the presence of type I and type II dipeptides (Turner et al., 2000, Nature, 405: 579-583).
The degradation signal for ubiquitylation via the N-end rule pathways is termed an N-degron and comprises the primary destabilizing N-terminal residue and an internal lysine which is the site of ubiquitylation. Destabilizing N-terminal residues can be generated through proteolytic cleavages of specific proteins and other N-terminal modifications which reveal destabilizing residues at the new N-terminus. The residues that are exposed or modified to reveal an N-degron have been termed a pre-N-degron or pro-N-degron. For example, Sindbis virus RNA polymerase is produced during viral infection through site-specific cleavage of the viral polyprotein precursor and carries an N-terminal Tyr that has been shown in rabbit reticulocyte lysates to target the protein for ubiquitylation via the N-end rule pathway (deGroot et al., 1991, Proc. Natl. Acad. Sci. U.S.A., 88:8967-8971). Another example is RGS4, whose N-terminal degradation signal is generated through a series of N-terminal modifications including (i) removal of N-terminal Met and exposure of Cys-2 at the N-terminus, (ii) oxidation of Cys-2 into cysteic acid, and (iii) conjugation of Arg to the N-terminus of the protein (Kwon et al., 2002, Science, 297: 96-99).
Very few N-end rule substrates are characterized to date. However, recently discovered N-end rule substrates linked to disease or pathology demonstrate the biological importance of N-end rule ubiquitylation pathway. For example, a carboxy-terminal fragment of cohesin in Saccharomyces cerevisiae is a physiological substrate for the ubiquitin/proteasome-dependent N-end rule pathway. Overexpression of this fragment is lethal and, in cells that lack an N-end rule ubiquitylation pathway, a highly increased frequency of chromosome loss is detected (Rao et al., 2001, Nature, 410: 955-959). Recent studies also indicate that enhanced protein breakdown in skeletal muscle leading to muscle wasting in patients with acute diabetes results from an accelerated Ub conjugation and protein degradation via the N-end rule pathway. The same pathway is activated in cancer cachexia, sepsis and hyperthyroidism (Lecher et al., 1999, J. Clin. Invest., 104: 1411-1420; Solomon et al., 1998, Proc. Natl. Acad. Sci. USA, 95: 12602-12607).
Aprataxin is a member of the HIT (histidine triad) protein family, named for the HφHφHφφ motif, where φ is a hydrophobic amino acid (Brenner, 2002, Biochemistry, 41(29): 9003-9014). HIT is a superfamily of nucleotide hydrolases and transferases, which act on the alpha-phosphate of ribonucleotides, and contain an approximately 30 kDa domain that is typically a homodimer of approximately 15 kDa polypeptides with two active sites. The superfamily is also generally said to include GalT-like proteins even though these contain a slightly different motif (HXHXQφφ), the motif being repeated twice in a single polypeptide chain that retains a single active site.
Members of the HIT superfamily of proteins have representatives in all cellular life. On the basis of sequence, substrate specificity, structure, evolution, and mechanism, HIT proteins can be classified into the Hint branch, which consists of adenosine 5′-monophosphoramide hydrolases, the Fhit branch, which consists of diadenosine polyphosphate hydrolases, and the GalT branch, which consists of specific nucleoside monophosphate transferases, including galactose-1-phosphate uridylyltransferase, diadenosine tetraphosphate phosphorylase, and adenylyl sulfate:phosphate adenylytransferase. A loss of at least one human representative of each branch is associated with a human disease (Brenner, 2002, Biochemistry, 41: 9003-9014). Fhit is lost early in the development of many epithelially derived tumors. GalT is deficient in galactosemia. Aprataxin, a Hint branch hydrolase, is mutated in ataxia-oculomotor apraxia syndrome (Date et al., 2001, Nature Genetics, 29: 184-188; Moreira et al., 2001, Nature Genetics, 29: 189-193), which is the most common autosomal recessive neurodegenerative disease among Europeans and people of European descent and the most frequent cause of autosomal recessive ataxia in Japan. Recent studies in patients with early-onset ataxia identified one insertion and two missense mutations in the aprataxin gene product (Shimazaki et al., 2002, Neurology, 59: 590-595). It has been suggested that aprataxin is involved in DNA repair and therefore its regulation is crucial for cancer predisposition and cerebellar neuron survival (Moreira et al., 2001, Nature Genetics, 29: 189-193; Durocher et al., 2000, Mol. Cell, 6: 1169-1182). The molecular targets and pathways involving aprataxin remain to be discovered. With identification of such targets and pathways, it is hoped that new light can be shed on brain development and motor coordination.
Microtubule-associated protein tau (MAPT or tau) is a protein that is believed to play a role in a variety of disease processes. The gene encoding tau undergoes complex, regulated alternative splicing, which gives rise to several mRNA species. Six tau isoforms are produced in adult human brain by alternative mRNA splicing from a single gene. The isoforms differ from each other by the presence or absence of 29-amino acid or 58-amino acid inserts located in the N-terminal half and a 31-amino repeat located in the C-terminal half. Inclusion of the latter, which is encoded by exon 10 of the tau gene, gives rise to three tau isoforms with four repeats each; the other three isoforms have three repeats each (Kosik et al., 1989, Neuron, 2: 1389-1397; Goedert et al., 1989, Neuron, 3: 519-526). The repeats and some adjoining sequences constitute the microtubule-binding domains of tau. Similar levels of 3-repeat and 4-repeat tau isoforms are found in normal cerebral cortex. The tau filaments from Alzheimer disease brain contain all six tau isoforms in a hyperphosphorylated state. At the same time, it is shown that the ratio of 3-repeat to 4-repeat tau isoforms is an important determinant of the ratio of microtubule-bound and free forms of tau (Lu et al., 2001, Mol. Biol. Cell, 12: 171-184). It has also been demonstrated that longer 4-repeat tau isoforms have approximately 4.5 times higher affinity to microtubules and 2-3 fold faster rate of microtubule assembly than 3-repeat tau isoforms (Goedart and Jakes, 1990, EMBO J, 9(13): 4225-4230; Butner and Kirschner, 1991, J. Cell Biol., 115(3): 717-730; Gustke et al., 1992, FEBS Lett., 307(2): 199-205).
MAPT transcripts (including splice isoforms and mutations) are differentially expressed in the nervous system, depending on the stage of neuronal maturation and neuron type. The shortest of the six tau isoforms termed 3R0N is specifically abundant in a fetal brain (Goedert et al., 1989, Neuron, 3(4): 519-526). In fact, the human brain expresses only 3R0N isoforms that are highly phosphorylated until the postnatal period, and this may imply a specific role of this isoform during axonal growth and synaptogenesis (Kosik et al., 1989, Neuron, 2(4): 1389-1397).
Mutations in tau result in several neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease, Pick's disease, frontotemporal dementia, cortico-basal degeneration and progressive supranuclear palsy as well as subcortical gliosis and pallido-nigro-luydian degeneration (Lu et al., 2001, Mol. Biol. of Cell, 12: 171-184; Golbe and Lazzarini, 2001, Mov. Disord., 16(3): 442-7 and references therein).
Neurofibrillary tangles made predominantly from intracellular bundles of self-assembled hyperphosphorylated tau proteins are the hallmark features of Alzheimer's disease (Mandelkow et al., 1995, Neurobiol Aging, 16(3): 347-354). Studies on the microtubule-associated protein tau in Alzheimer disease have noted that, in the brains of patients with Alzheimer disease, the neuronal cytoskeleton is progressively disrupted and replaced by neurofibrillary tangles of paired helical filaments (PHFs), composed mainly of hyperphosphorylated forms of tau (also called ‘AD P-tau’). It has also been demonstrated that in solution normal tau associates with the hyperphosphorylated AD P-tau to form large tangles of filaments and that dephosphorylation with alkaline phosphatase abolished the ability of AD P-tau to aggregate in vitro (Johnson and Bailey, 2002, J. Alzheimer Disease, 4: 375-398).
It has also been shown that elevated levels of tau inhibit intracellular transport in neurons, particularly the plus-end-directed transport by kinesin motors from the center of the cell body to the neuronal processes (Ebneth et al., 1998, J. Cell Biol., 143(3): 777-794). This inhibition is significant because critical organelles, such as peroxisomes, mitochondria, and transport vesicles carrying supplies for the growth cone, are unable to penetrate the neurites, leading to stunted growth, increased susceptibility to oxidative stress, and likely pathologic aggregation of proteins such as amyloid precursor protein. It has been concluded that the tau:tubulin ratio is normally low, and that increased levels of tau become detrimental to the cell (Ebneth et al., 1998, J. Cell Biol., 143(3): 777-794).
Synaptotagmin-like proteins (SLPs) are a subfamily of the C2 domain-containing family of proteins and have a high degree of homology to synaptotagmin. The proteins contain two conserved domains at the N-terminus (referred to as SLP homology domains 1 and 2 or SHD1 and 2) and two carboxyl-terminal Ca2+-binding motifs (C2 domains) (Pallanck, 2003, TRENDS Neurosci., 26(1): 2-4). The SHD has also been found in other proteins including SLP homologs lacking C2 domains (Slac2). Found in phospholipases and protein kinase C, C2 domains have also been identified in synaptotagmins, a family of proteins involved in regulating neurotransmission. The function of synaptotagmin, as a calcium sensor in SNARE-mediated exocytosis, is extremely complex and finely regulated to allow for coupling the calcium signal to the fast synaptic vesicle exocytosis, which leads to speculations that synaptotagmin evolved to acquire a function beyond calcium/phospholipids binding (Rickman and Davletov, J. Biol. Chem., 2002, [epub]; Yoshihara et al., 2002, Neuron, 36: 897-908; O'Connor et al., 2002, Nature Neurosci., 5(9): 823-824). Also the downregulation of synaptotagmin expression in cholinergic neurons of the nucleus basalis in patients with Alzheimer's disease was reported (Mufson et al., 2002, Neurochem. Res., 27(10): 1035-1048), this downregulation is highly specific, as no downregulation was observed for synapsin I, synaptobrevin or SNAP-29 in the same study.
Like synaptotagmin, the SLP and Slac2 proteins are also believed to play a role in regulation of vesicular trafficking (see Strom et al., 2002, J. Biol. Chem., 277: 25423-25430 and Kuroda et al., J. Biol. Chem., 277: 9212-9218 for a description of the role of SHD containing proteins and their relevance to disease states). The SHD is a binding domain for the GTP-bound form of Rab27a, one of the small GTP-binding proteins that are believed to be essential components of the membrane trafficking mechanism of eukaryotic cells (Zerial et al., 2001, Nat. Rev. Mol. Cell. Biol. 2, 107-117). The C-terminal domains of the SLP and Slac2 proteins are likely to play a role in the localization Rab27a to specific sites in a cell.
Rab27a is involved in the transport of melanosomes in melanocytes and lytic granules in cytotoxic T-lymphocytes. Griscelli syndrome, a disease caused by a mutation in Rab27 which leads to defects in the transport of melanosomes and lytic granules is characterized by partial albinism and severe immunodeficiency with hemophagocytic syndrome. Overexpression of the SHD sequence led to a dominant negative effect and a defect in the transport of melanosomes in melanocytes that was similar to that observed in Griscelli syndrome patients.
Alternative splicing occurs at the C2 domain locus and variants of the synaptotagmin-like proteins have been identified. Additional splice variants have been suggested but supporting sequence confirmation is not yet available. Also a gene encoding a synaptotagmin-like protein contains a region of weak similarity to murine Gph.
High Mobility Group Chromosomal Protein HMG17 (also known as HMGN2) is a member of the HMG 14/17 (also known as HMGN) family of proteins; which bind DNA with low specificity and share a common DNA-binding motif with members of the HMG 1/2 (also known as HMGB) family of proteins.
Chromosomal proteins HMG-17 and HMG-14 are among the most abundant, ubiquitous, and evolutionarily conserved nonhistone proteins found in the nuclei of higher eukaryotes (Landsman et al., 1986, J. Biol. Chem., 261(16): 7479-7484). A large number of retropseudogenes are scattered over several chromosomes. It has been shown that the nonhistone chromosomal proteins HMG-14 and HMG-17 are encoded by distinct genes, each of which is part of a separate multigene family. These families may have evolved independently from similar genetic elements or from a shared ancestral gene in which the nucleotide sequence coding for the DNA-binding domain of the protein is the most conserved region. The structural differences between the molecules and the differences in their DNA-binding domains suggest that the proteins may be involved in distinguishable cellular functions. It was suggested that they may confer specific conformations to transcriptionally active regions of chromatin (Weisbrod and Weintraub, 1979, Proc. Natl. Acad. Sci. USA, 76: 630) thereby changing the transcriptional potential of the chromatin template (Almouzni et al., 1990, EMBO J., 9: 573; Svaren and Chalkley, 1990, Trends Genet, 6: 52). Microinjections of antibodies to HMG-17 into human fibroblasts inhibited transcription (Einck and Bustin, 1983, Proc. Natl. Acad. Sci. USA, 80: 6735). Some data suggest that HMG-17 binds to chromatin in a tissue specific manner (Brotherton et al., 1990, Nucl. Acids Res., 18: 2011).
The putative role of HMG-17 in chromatin structure and gene expression is supported by its differential expression during cell differentiation (reviewed in Bustin et al., 1992, CRC Crit. Rev. Eukaryotic Gene Expression, 2: 137). Analyses of the mRNA levels during the course of erythropoiesis (Crippa et al., 1991, J. Biol. Chem., 266: 2712), myogenesis (Pash et al., 1990, J. Biol. Chem., 265: 4197), osteoblast differentiation of several additional cell lines (Crippa et al., 1990, Cancer Res., 50: 2022) indicate that undifferentiated cell synthesize more HMG mRNA than differentiated cells. Results suggest that myogenic differentiation may require regulated levels of HMG-14 (Pash et al., 1993, J. Biol. Chem., 268: 13632) and that HMG-14 mRNA and protein levels are elevated in tissues from the individuals with Down syndrome (Pash et al., 1991, Exp. Cell Res., 193: 232) and in trisomy-16 mouse (Pash et al., 1993, J. Biol. Chem., 268: 13632).
A 31-amino acid synthetic peptide from HMG17, when injected intravenously, accumulates in the nuclei of tumor endothelial cells and tumor cells and can carry a “payload” such as a fluorescent label to a tumor and into the cell nuclei in the tumor. This result suggests that HMGN2, like HMGB1, may have a role as an extra-cellular signaling molecule (Porkka et al., 2002, Proc. Natl. Acad. Sci. USA, 99: 7444-7449).
PIN2-interacting protein 1 (PinX1) is an RNA processing protein, which contains a G-patch domain (Aravind and Koonin, 1999, Trends Biochem Sci., 24: 342-344). The 328-amino acid length of PinX1 protein is predicted from a longest polynucleotide sequence of PINX1 identified in a yeast 2-hybrid assay. The protein contains no known domain structure except for a gly-rich region in its N-terminus and a telomerase (TERT) inhibitory domain (TID) in its C terminus. The prediction is confirmed by a Northern blot analysis which has detected a 1.9-kb PINX1 transcript in all tissues tested. Immunoprecipitation and immunoblot analyses indicate that PINX1 encodes a 45-kD protein in cells (Zhou and Lu, 2001, Cell, 107: 347-359).
PinX1 is identified as a Pin2 binding protein in a yeast 2-hybrid assay and confirmed in coimmunoprecipitation, colocalization and pull-down experiments (Zhou and Lu, 2001, Cell, 107: 347-359). It is also discovered that PINX1 inhibits telomerase activity and affect tumorigenicity, when the small TID domain of PinX1 binds the telomerase catalytic subunit hTERT and potently inhibits hTERT activity (Zhou and Lu, 2001, Cell, 107: 347-359; Kishi and Lu, 2002, J. Biol. Chem., 277(9): 7420-7429).
Telomerase activity is important for normal and transformed human cells and is implicated in oncogenesis. Overexpression of PinX1 or its TID domain inhibits telomerase activity, shortens telomeres, and induces crisis, whereas depletion of endogenous PinX1 increases telomerase activity and elongates telomeres. Depletion of PinX1 also increases tumorigenicity in nude mice, consistent with its chromosome localization at 8p23, a region with frequent loss of heterozygosity in a number of human cancers. Thus, PinX1 is a potent telomerase inhibitor and a putative tumor suppressor (Zhou and Lu, 2001, Cell, 107: 347-359; Kishi and Lu, 2002, J. Biol. Chem., 277(9): 7420-7429).
Unlike human PinX1, which inhibits telomerase activity, the putative yeast homolog of PinX1, encoded by the YGR280c open reading frame (ORF), is a component of the ribosomal RNA processing machinery involved in rRNA and small nucleolar RNA maturation (Guglielmi and Werner, 2002, J. Biol. Chem., 277(38): 35712-35719). The protein has a KK(E/D) C-terminal domain typical of nucleolar proteins and a putative RNA interacting domain widespread in eukaryotes called the G-patch. The protein is hence renamed Gno1p (G-patch nucleolar protein). GNO1 deletion results in a large growth defect due to the inhibition of the pre-ribosomal RNA processing first cleavage steps at sites A(0), A(1), and A(2). Furthermore, Gno1p is involved in the final 3′-end trimming of U18 and U24 small nucleolar RNAs. Mutational analysis shows that the G-patch of Gno1p is essential for both functions, whereas the KK(E/D) repeats are only required for U18 small nucleolar RNA maturation.
Human PinX1 expression in the yeast multicopy expression vector pGen in the background of gno1-Delta (deletion of GNO1 gene) phenotype suggests that it has a dual function in telomere length regulation and ribosomal RNA maturation in agreement with its telomeric and nucleolar localization reported in human cells (Guglielmi and Werner, 2002, J Biol Chem, 277(38): 35712-35719). Conversely, the same study finds that a full length yeast Gno1p does not exhibit the in vivo telomerase inhibitory activity of PinX1.
CBF1-interacting corepressor (CIR) is a unique CBF1 interacting corepressor, which binds to histone deacetylase and to SAP30 and serves as a linker between CBF1 and the histone deacetylase complex (Hsieh et al., 1999, Proc Nat Acad Sci, 96: 23-28). CBF1 (RBPSUh) is a member of the CSL family of DNA-binding factors, which mediate transcriptional activation or repression. The family includes CBF1, ‘suppressor of hairless,’ and Lag1 (Schweisguth and Posakony, 1992, Cell, 69: 1199-1212; Christensen et al., 1996, Development (Cambridge, UK), 122: 1373-1383). CSL proteins play a central role in Notch signaling (Artavanis-Tsakonas et al., 1995, Science, 268: 225-232). Disruptions and aberrations in Notch signaling are associated with human neoplastic disease, cerebral autosomal dominant arteriopathy with subcortical infarcts and leucoencepholopathy, and Alagille syndrome. CSL proteins also play a central role in and in Epstein-Barr virus-induced immortalization (Zhou et al., 2000, J. Virol., 74(4): 1939-1947; Hsieh et al., 1999, Proc. Natl. Acad Sci. USA, 96: 23-28 and references therein), the process associated with human malignancies such as Burkitt's lymphoma, nasopharyngeal carcinoma, Hodgkin's disease and lymphoprolifirative disease in immunosuppressed patients. CIR is believed to play an important role in the gene regulation activity of CBF1 by targeting CBF1 recognition sequences for histone deacetylation, an activity which is linked to gene suppression.
Human CIR is a 450-amino acid protein (560-amino acid C. elegans homolog) with a highly charged, serine-rich C-terminus predicted from a polynucleotide sequence identified using a yeast 2-hybrid screen of a B-cell cDNA library with CBF1 as bait (Hsieh et al., 1999, Proc. Natl. Acad. Sci. USA, 96: 23-28). The widespread expression of CIR is revealed by Northern blot analysis, with strongest expression detected in heart, skeletal muscle, and pancreas. Immunofluorescence analysis shows that CIR is a nuclear protein like CBF1, although CIR does not bind to the nucleolus. It is determined that the N-terminal 121 amino acids of CIR interact with amino acids 233 to 249 of CBF1 and repress transcriptional activity. Yeast two-hybrid assay and immunofluorescence analysis indicate that CIR also interacts with HDAC2 and SAP30, important mediators of transcriptional repression.
Cullin 3, a member of the cullin/Cdc53 family of proteins, is a putative E3 ubiquitin ligase which regulates abundance of other proteins, such as Ci (Ou et al., 2002, Genes Dev., 16(18): 2413-2414), cyclins E and D1 (Singer et al., 1999, Genes Dev., 13: 2375-2387; Winston et al., 1999, Genes Dev., 13: 2351-2357; Maeda et al., 2001, FEBS Lett., 494: 181-185) and katanin (Kurtz et al., 2002, Science, 295: 1294-1298) by targeting them for ubiquitin-dependent proteolysis, potentially as a part of a larger complex.
Ci protein is required for cell's ability to sense and interpret graded spatial information and therefore ultimately responsible for cell's fate (Ou et al., 2002, Genes Dev., 16(18): 2413-2414; Jiang J, 2002, Genes Dev., 16: 2315-2321). Overexpression of cyclin D1 has been implicated in a variety of tumors such as breast cancers, gastrointestinal cancers and lymphomas (Maeda et al., 2001, FEBS Lett., 494: 181-185). Cyclin E is an evolutionary conserved protein whose essential function is to promote cell cycle transition from G1 to S thereby coordinating crucial events in the organism (Knoblich et al., 1994, Cell, 77: 107-120; Ohtsubo et al., 1995, Mol. Cell. Biol., 15: 2612-2624). Katanin, which has documented microtubule-severing activity, regulates microtubule instability and the loss of katanin completely suppresses all signs of instability (Han et al, 1997, Nature, 386: 296; Kurtz et al., 2002, Science, 295: 1294-1298) in C. elegans. 
The cullins represent a conserved gene family, with at least five members in nematodes, six in humans, and three in S. cerevisiae. Human CUL3 is an ortholog of nematode cul3 (Winston et al., 1999, Genes Dev., 13: 2351-2357) which is expressed in several tissues as major 2.8- and minor 4.3-kb transcripts in various tissues, with the highest levels in skeletal muscle and heart. CUL3 has been identified as a gene whose expression in human fibroblasts is induced by phorbol 12-myristate 13-acetate (PMA) and suppressed by salicylate. Homozygous deletion of the Cul-3 gene is shown to cause embryonic lethal phenotype (Singer et al., 1999, Genes Dev., 13: 2375-2387).
The methods developed for monitoring the ubiquitylation activity of cullins, or modulating such activity are described in U.S. Pat. No. 6,426,205 to Tyers et al., and U.S. Pat. Nos. 6,165,731 and 6,413,725 to Deshaies et al., hereby incorporated by reference.
High-mobility group protein HMGN3, also known as a thyroid hormone receptor interacting protein 7 (Trip7), is a member of a class of relatively abundant non-histone nuclear proteins, which function as architectural elements (Bustin and Reeves, 1996, In Cohn, Moldave eds., Progress in Nucleic Acid Research and Molecular Biology, Vol. 54, San Diego, Academic Press, 35-100). HMGN3 (Trip7) is closely related to the small nonhistone chromatin proteins HMG14 and HMG17 (HMGN2), which bind specifically to nucleosomes, reduce the compactness of chromatin fiber and enhance transcription from chromatin templates (Bustin, 2001, Trends Biochem. Sci., 26: 431-437). In addition, a yeast two-hybrid screen in a HeLa cell library indicated that HMGN3 interacts with the ligand binding domain of thyroid hormone receptor beta (TRβ1), but only in the presence of thyroid hormone (Lee et al., 1995, Mol. Endocrinol., 9: 243-254).
The thyroid hormone receptors (TRs) are hormone-dependent transcription factors that regulate expression of a variety of specific target genes. It is suggested that they are regulated by a number of proteins as they progress from their initial translation and nuclear translocation to heterodimerization with retinoid X receptors (RXRs) and further to functional interactions with other transcription factors and the basic transcriptional apparatus, and eventually, degradation (Collingwood et al., 1999, J. Mol. Endocrinol, 23: 255-275). Interestingly, all Trips (including HMGN3) interact with RXR-alpha (RXRA) in a ligand-dependent manner, but HMGN3 does not interact with the glucocorticoid receptor (NR3C1) under any conditions (Lee et al., 1995, Mol. Endocrinol., 9: 243-254).
Northern blot analysis detects a 1.1-kb TRIP7 transcript in several tissues, with highest expression in heart and kidney (Lee et al., 1995, Mol. Endocrinol., 9: 243-254). Both Northern and Western analysis demonstrate a tissue specific expression pattern in mice with the highest level of expression in whole mouse brain extracts (West et al., 2001, J. Biol. Chem., 276: 25959-25969). The additional immunohistochemical data reveals that the expression of HMGN3 is enriched in specific regions of the mouse brain with relatively high expression in lateral olfactory tract, anterior commissure, corpus callosum, internal capsule, fornix, stria medullans, optic tract and axon bundles (Ito and Bustin, 2002, J. Histochem. Cytochem., 50(9): 1273-1275). The expression pattern most closely resembles the expression pattern of GFAP (glial fibrillary acidic protein) which is considered an important factor in astrocyte differentiation and is part of the reactive response of the CNS to injury (Eng et al., 2000, Neurochem. Res., 25: 1439-1451). The results raise the possibility that HMGN3 protein plays a functional role in the astrocytes of mouse brain (Ito and Bustin, 2002, J. Histochem. Cytochem., 50(9): 1273-1275).
A separate study of an HMGN3 (Trip7) homolog in Xenopus laevis implicated the protein in tissue remodeling during the metamorphosis. The study shows that HMGN3 influences basal transcription in a chromatin structure-dependent manner, but enhances the function of liganded TR regardless of the chromatin structure of the promoter (Amano et al., 2002, Developmental Dynamics, 223: 526-535).
HSPC144 protein is a homolog of the chicken thymocyte protein (cThy28), a protein that is suggested to mediate avian lymphocyte apoptosis (Compton et al., 2001, Apoptosis, 6: 299-314). Avian cThy28 gene is a 1070 bp cDNA encoding a 242 amino acid conserved protein, cThy28 (GenBank accession number U34350) that shares greater than 90% amino acid similarity with several putative mammalian homologues such as mouse mThy28 (226 aa) (Miyaji et al., 2002, Gene, 297: 189-196) and a human HSPC144 (225 aa) obtained from a human CD34+ stem cell library. A structural analysis of the protein suggests that it is a nuclear-localized phosphoprotein with potential glycosylation and myristolation sites. Compared to other non-lymphoid tissues, the avian cThy28 protein and its transcript are present in immune organs at elevated levels. The mouse homolog is expressed in testis, liver, brain and kidney with the lower levels of expression in thymus spleen, heart and stomach (Miyaji et al., 2002, Gene, 297: 189-196). The high degree of conservation in amino acid sequences among various species including bacteria, yeast and plants as well as vertebrate suggests an indispensable role of the protein in living cells.
Human Cell Cycle Controller CDC6 protein, a protein that is highly similar to Saccharomyces cerevisiae Cdc6 protein, is essential for the initiation of DNA replication (for review see, Bell and Dutta, 2002, Annu. Rev. Biochem., 71: 333-374; Lee and Bell, 2000, Curr. Opin. Cell Biol., 12: 280-285; Lei and Tye, 2001, J. Cell Sci., 114: 1447-1454; Coleman, 2002, Curr. Biol., 12(22): R759). Cdc6 is part of a macromolecular machine that assembles on chromatin to target the DNA for a single round of replication. Cdc6 (Saccharomyces cerevisiae) and Cdc18 (Schizosaccharomyces pombe) collaborate with the six-subunit origin recognition complex (ORC, Orc1) (Cocker et al., 1996, Nature, 379: 180-182; Tanaka et al., 1997, Cell, 90: 649-660), Cdt1 (Maiorano et al., 2000, Nature, 404: 622-625; Nishitani et al., 2000, Nature, 404: 625-628) and with the DNA replication proteins (PCNA, RPA) to recruit minichromosome maintenance (MCM) family of proteins to DNA, thereby forming the pre-replication complex.
Human CDC6 localizes in cell nucleus during cell cycle G1, but translocates to the cytoplasm at the start of S phase, suggesting that DNA replication may be regulated by either the translocation of this protein between the nucleus and cytoplasm or by selective degradation of the protein in the nucleus, as revealed by immunofluorescent analysis of epitope-tagged protein (Delmolino et al., 2001, J. Biol. Chem., 276(29): 26947-26954). The subcellular translocation of Cdc6 during cell cycle is also regulated through its phosphorylation by Cdks at nuclear localization signals or at nuclear export signals or at sites adjacent to these (Delmolino et al., 2001, J. Biol. Chem., 276(29): 26947-26954).
There is evidence that human Cdc6 regulates the onset of mitosis, as overexpression of human Cdc6 in G2 phase cells prevents entry into mitosis by blocking cells in G2 phase via a checkpoint pathway involving Chk1 (Clay-Farrace et al., 2003, EMBO J., 22(3): 704-712). Transcription of human Cdc6 is regulated in response to mitogenic signals through transcriptional control mechanism involving E2F proteins, as revealed by a functional analysis of the human Cdc6 promoter and by the ability of exogenously expressed E2F proteins to stimulate the endogenous Cdc6 gene (Ohtani et al., 1998, Oncogene, 17: 1777-1785; Hateboer et al., 1998, Mol. Cell. Biol., 18(11): 6679-6697).
Northern blots indicate that CDC6/Cdc18 mRNA levels peak at the onset of S phase and diminish at the onset of mitosis in HeLa cells, but total CDC6/Cdc18 protein level is unchanged throughout the cell cycle. Immunoprecipitation studies show that human CDC6/Cdc18 associates in vivo with cyclins (such as cyclin A and B), CDK 1 and/or CDK2, and ORC1 (Clay-Farrace et al., 2003, EMBO J., 22(3): 704-712; Yam et al., 2002, Cell Mol. Life Sci., 59: 1317-1326). The association of cyclin-CDK2 with CDC6/Cdc18 is specifically inhibited by a factor present in mitotic cell extracts. It has been suggested that if the interaction between CDC6/Cdc18 with the S phase-promoting factor cyclin-CDK2 is essential for the initiation of DNA replication, the mitotic inhibitor of this interaction could prevent a premature interaction until the appropriate time in G1 (for review see, Yam et al., 2002, Cell Mol. Life. Sci., 59: 1317-1326).
Cdc6 is expressed selectively in proliferating but not quiescent mammalian cells, both in culture and within tissues in intact animals. For example, nuclear human Cdc6 is detected by immunofluorescence in 90% of nuclei in premalignant human cervical tissue (Williams et al., 1998, Proc. Natl. Acad. Sci. USA, 95: 14932-14937). Most studies agree that markers of proliferation correlate with patient prognosis (Gerdes et al., 1983, Intl. J. Cancer, 31: 13-20; Thapar et al., 1996, Neurosurgery, 38: 99-106). The expression of Cdc6 may be used as marker of proliferating cells in various types of tumors (Freeman et al., 1999, Clin. Cancer. Res., 5: 2121-2132; Ohta et al., 2001, Oncology Reports, 8: 1063-1066). During the transition from a growth-arrested to a proliferative state, transcription of mammalian Cdc6 is regulated by E2F proteins (Ohtani et al., 1998, Oncogene, 17: 1777-1785). For example, a significant downregulation of Cdc6 expression found in prostate cancer is attributed to E2F and Oct1 transcription factors (Robles et al., 2002, J. Biol. Chem., 277(28): 25431-25438). Immunodepletion of Cdc6 by microinjection of anti-Cdc6 antibody blocks initiation of DNA replication in human tumors including tumors of neuroepithelial tissue, vestibular schwannomas, meningiomas and plurality adenomas, or human tumor cell lines (Ohta et al., 2001, Oncology Reports, 8: 1063-1066).
The additional regulatory event controlling initiation of DNA replication in mammalian cells is hypothesized to be dephosphorylation of CDC6 by PP2A, mediated by a specific interaction with PR48 or a related B″ protein (Yan et al., 2000, Mol. Cell Biol., 20(3): 1021-1029). The study demonstrates that an N-terminal segment of CDC6 binds specifically to PR48, a regulatory subunit of protein phosphatase 2A (PP2A).